This invention relates to method and system for controlling a machine tool, such as a turning machine.
The control of machine tools, such as turning machines, has evolved greatly over the past decade or so. However, one constant has remained, that is, the requirement for correlating the cutting tool's radial position with the rotation of the part to produce a desired shape. This correlation was provided in some early turning machines by a mechanical cam and follower system. However, this required the actual turning of a template workpiece. This prevented the simplified adaptability of such a machine tool when different shaped workpieces, such as pistons, were to be turned.
The general idea of applying a CNC system to a cutting tool subsequently evolved. In such a system, a computer and a numerical control with a feedback loop compared a position signal representing the present position of the tool to a program position signal to produce an error signal which controlled energization of a linear motor driving the tool.
The use of a CNC system allows parts, such as pistons, having an elliptical cross-sectional shape to be turned at greater than 1,200 rpm. In such a system the cutting tool must make tool reciprocations radially of the part for each complete revolution of the part. In the case of a piston rotating at 2,000 rpm this requires that the cutting tool execute precisely controlled oscillations at a frequency of 80 hz. In the case of a piston rotating at 3,600 rpm, the cutting tool must oscillate at a frequency of 120 hz. At 2,400 rpm, if a 0.015 inch radial displacement is required, the corresponding acceleration amounts to 315 feet/second.sup.2.
The control of the oscillating cutting tool is complicated by the load imposed by the interaction of the cutting tool with the rotating part. Consequently, the cutting tool must be constructed and supported to react to the loads, such as inertia, friction and transient cutting forces, without undesired side effects, such as tool chatter and/or deflection, so that the desired contour of the part can be achieved.
The control system of such a turning machine preferably should have: a steady state accuracy of 20 micro inches or better; an overshoot to a step command of less than 5%; a stiffness of no less than 300,000 lbs. per inch; and an update rate of once for every degree of workpiece rotation at 1,800 rpm (i.e. every 92 micro seconds).
The prior art includes a CNC turning machine, including a CNC system wherein closed loop control of the turning operation is performed on a part such as an elliptical piston. The relative axial position of the part to the cutting tool and the rotary position of the part about the axis of rotation are precisely controlled and known at all times. The CNC acts upon a part program in conjunction with the closed loop control to issue correlated commands for use in controlling a voice coil motor and, hence, the radial oscillations of a cutting tool coupled thereto. Commands are transmitted via a highspeed data link to a position profile computer which translates the commands into an appropriate form causing the voice coil motor to produce a double oscillation of the cutting tool for each revolution of the part. The position profile computer is dedicated to the radial position of the cutting tool and forms a portion of the closed loop control of the cutting tool position. Various sensors, including a velocity transducer, provide feedback signals to the closed loop control wherein the loop is closed in hardware. U.S. Pat. No. 4,653,360 entitled "CNC Turning Machine" and issued Mar. 31, 1987 is an example of such prior art.
The speed of digital signal processing microprocessors such as the Texas Instrument TMS 320 family of microprocessors and systems utilizing the LM628 chip from National Semiconductor of Santa Clara, Calif. have allowed the use of complex algorithms on microprocessor-based control systems. The feasibility of any such control system is a function of the number of mathematical operations involved in the control program difference equation(s), and especially the number of multiplications.
A digital filter is a circuit or computer program that may be both linear and time-invariant and operates on discrete time signals. Digital filters can be built with conventional digital hardware or can also be implemented as digital programs on suitable general purpose or special purpose computers or microcomputers.
There are two basic types of digital filters, recursive or infinite-impulse-response filters and non-recursive or finite-impulse-response filters. Causal varieties of these filters can be developed from continuous time functions in many well-known fashions.